Development of solid polymer fuel cells that employ alcohol fuel is being actively promoted for use as a power source for various electronic apparatuses including mobile phones, because such cells are easy to be made smaller in dimensions and lighter in weight. The development is primarily focused on solid polymer fuel cells that employ methanol fuel, owing to higher energy density thereby attained.
The solid polymer fuel cell includes a Membrane and Electrode Assembly (hereinafter, MEA), in which a solid polymer electrolytic membrane is interleaved between an anode and a cathode. The fuel cell that directly supplies the liquid fuel to the anode is called a direct-type fuel cell, in which the supplied liquid fuel is decomposed on a catalyst carried by the anode, so that positive ion, electron and an intermediate product are given. In the fuel cell of this type, the positive ion thus generated further migrates to the cathode through the solid polymer electrolytic membrane, while the generated electron migrates to the cathode through an external load, to be reacted with oxygen in the atmosphere on the cathode, thereby generating electricity. In the direct methanol fuel cell (hereinafter, DMFC) that employs, for example, methanol aqueous solution as it is as the liquid fuel, the reaction represented by the chemical formula (1) takes place on the anode, and the reaction represented by the formula (2) takes place on the cathode. As is apparent from these formulae (1) and (2), theoretically 1 mol of methanol and 1 mol of water are reacted on the anode to thereby give 1 mol of reaction product (carbon dioxide) on the DMFC, and since hydrogen ion and electron are also generated simultaneously, the theoretical concentration of methanol in the methanol aqueous solution, serving as the fuel, is approx. 70% in volume (vol. %).CH3OH+H2O→CO2+6H++6e−  (1)6H++6e−+3/2O2→3H2O  (2)
It is known, however, that in the case where the fuel concentration becomes higher and hence a relatively larger amount of alcohol fuel is supplied to the anode than water, what is known as “cross-over effect” takes place in which the alcohol fuel is transmitted through the solid polymer electrolytic membrane without being involved in the reaction represented by the formula (1), to be reacted with the catalyst on the cathode, which results in decreased generation capacity and generation efficiency.
Examples of techniques that suppress the cross-over effect include providing a fuel vaporization layer constituted of a porous material or the like that vaporizes the liquid fuel on the upstream side of the anode of the MEA, to thereby supply the given vaporized liquid fuel (Ref. patent document 1). The patent document 1 states the advantage thereof as “Supplying thus the vaporized fuel allows maintaining the gas fuel in the fuel vaporization layer substantially saturated, and the liquid fuel is vaporized in the amount corresponding to the consumption of the gas fuel in the fuel vaporization layer for the cell reaction, and then the liquid fuel of the amount corresponding to the vaporized amount is introduced into the cell via capillary effect. Thus, since the fuel supply amount is linked with the fuel consumption, the fuel is scarcely discharged unreacted out of the cell, which does not require providing a processing system on the fuel outlet side as a conventional liquid fuel cell.”
Also, the present inventors have discovered the drawback incidental to the patent document 1, in that the CO2 gas generated on the anode resides between the anode and the gas-liquid separation membrane, thereby impeding stable generation of power, and have proposed, as the solution thereof, a fuel cell that includes an outlet on a sealing material on the lateral side of the anode that facilitates efficiently discharging the generated CO2 gas, and a system including such fuel cell (Ref. patent document 2).
Another drawback has come up, that in the case where the methanol concentration in the fuel is increased, water supply for the methanol becomes insufficient on the anode, which disables increasing the output. The present inventors have therefore developed a fuel cell that returns the water, generated on the cathode as shown by the formula (2), to the anode for utilization, and a system that employs such fuel cell. The present inventors have thus proposed, for example, a fuel cell and a fuel cell system having a MEA structure that includes a water repellent porous material on the cathode to thereby suppress the transpiration of the water generated on the cathode. Such structure allows a part of the water generated on the cathode to return to the anode, so that the shortage of water on the anode is resolved and thereby the output is increased.
Thus, employing the technique of vaporizing the liquid fuel and suppressing the transpiration of the water generated on the cathode enables attaining a high generation characteristic despite employing a high-concentration methanol fuel.
[Patent document 1] JP-A No. 2000-106201
[Patent document 2] JP-A No. 2006-318708